Process for preparing finely divided hydrophobic oxide particles

ABSTRACT

A process for preparing hydrophobic finely divided particles of oxides of metals and/or oxides of silicon by chemically bonding hydrocarbon radicals to the surface of the oxide particles, said process comprising intensively mixing oxide particles having a water content less than about 1% by weight with a hydrolyzable metal or metalloid compound containing OR groups, wherein said compound is an ester of the general formula: 
     
         Me(OR).sub.4 
    
     wherein Me is an element from group IV of the periodic table, and each R is independently selected from C 1  -C 8  alkyl radicals or an aryl radical. A hydrophobic silicon dioxide particle is provided. Particles prepared by the process are useful as fillers in elastomers.

CROSS-REFERENCES TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 201,320, filed Nov. 23,1971, now abandoned, which is relied upon and the entire disclosure andspecification of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

This invention relates to a process for preparing hydrophobic finelydivided particles of oxides of metals and oxides of silicon bychemically bonding hydrocarbon radicals to the surface of the oxideparticles. More particularly, the oxides are treated with hydrolysablemetal or metalloid compounds containing OR groups.

It is known in the art that particles can be rendered hydrophobic bytreatment with organo -silanes to form metals or metalloids on thesurface of the particles. Such products are particularly useful when thehydrocarbon radicals (preferably methyl) are chemically bonded to thesurface of finely distributed oxides, which renders the oxides,depending upon the quantity of hydrocarbon radicals present, eitherpartially or completely hydrophobic.

It is characteristic of this type of treated particle that thetransition from entirely hydrophilic to entirely hydrophobic behaviorfirst occurs with a certain "degree of hydrophobing", more or lessimmediately without any distinct intermediate phase. Thus for example,products of this type with only 0.3% C or less possess hydrophiliccharacteristics and behave like the untreated materials. On the otherhand, products with at least about 0.5 - 0.7% C are practically entirelyhydrophobic depending on the specific surface characteristics of thestarting oxides. It would be desirable to be able to prepare productshaving a degree of hydrophobing lying between these extremes, whichbehave like mixtures of hydrophobic and hydrophilic oxides. However,experience has shown that it is not possible to obtain products whichare hydrophobic and cannot be wetted with water, but which neverthelessretain certain hydrophilic characteristics. For certain purposes, forexample, reinforcing agents for silicon rubber and other elastomers, itis desirable to have fillers which are hydrophobic and distinctlyorganophilic, but which also have an affinity for protons. Thepreviously mentioned hydrophobic products do not possess this lattercharacteristic since the alkyl or aryl radicals, bonded to siliconatoms, possess no affinity for protons. Also, small residues of, forexample, silanol groups, on the oxide particles (such as occur in atruly hydrophobic product) are screened by large numbers of alkyl groupsin such a way that the particle does not exhibit proton affinity. Thislatter effect has actually become a quality characteristic of suchproducts.

Clearly, entirely hydrophobic oxides are not wetted by water. However,finely divided hydrophobic silicon dioxide particles can demonstrateproton affinity by treating the particles in such a way that only partof the silanol groups located on the surface are converted to siliconalkyl groups, while the remainder of the silanol groups are convertedto, for example, silicon alkoxy groups. Unlike the silicon alkyl group,the silicon alkoxy group exhibits proton affinity to a certain degree.

It is known in the art that such products can be produced by treatingsilicon dioxide with organoalkoxy silanes. The quantity ratio of siliconalkyl groups to silicon alkoxy groups located on the surface of theparticles is equal to or greater than 1.

Particles, such as fillers, which are to be lipophilic, but which on theother hand are to possess a high degree of proton affinity withsimultaneous nonwettability by water, must have as many silicon alkoxygroups as possible, and as few silanol groups as possible. Preferablythe particles contain no silanol groups at all. The number of siloconalkyl groups is only of secondary importance, and in some cases of noimportance at all. There presently exists a gap in the state of the artby which such products can be obtained.

It is known in the art that silicic acid aerogels can be givenorganophilic characteristics by coating them with a surface film of apolymeric organic silicate. (See DAS No. 1,048,889). These products aredefined in part by the degree of polymerization of the surface film. Ifthe degree of polymerization of the surface film lies above 25, theproduct can be hydrophilic, partially hydrophobic, but not organophilicor hydrophobic and organophilic. Since one is not dealing with uniformlyhydrophobic material, but rather with hydrophilic particles wrapped inan organic more or less water repellant, layer wettable by water,considerable quantities of organic "wrapping material" are needed. Forexample, in order to obtain only partially hydrophobic products, about20% by weight tetraethyl-alcosilicate is required. It will be apparantthat in order to achieve completely hydrophobic products,disproportionately larger quantities of "wrapping material" are needed.For this reason, the production of entirely hydrophobic products by thismethod is impractical. (See the above DAS)

BRIEF SUMMARY OF THE INVENTION

According to this invention there is provided a process for preparinghydrophobic finely divided particles of oxides of metals and/or oxidesof silicon by chemically bonding hydrocarbon radicals to the surface ofthe oxide particles. The process comprises intensively mixing oxideparticles having a water content less than about 1% by weight with ahydrolysable metal or metalloid compound containing OR groups. Thecompound is an ester of the general formula:

    Me(OR).sub.4

wherein Me is an element from group IV of the periodic table, and each Ris independently selected from C₁ -C₈ alkyl radicals or an aryl radicalor an alkyl-aryl radical.

This invention also provides a finely divided hydrophobic and lipophilicsilicon dioxide particle practically free of silanol groups on itssurface, said particle having chemically bonded thereto alkoxy oraryloxy groups sufficient to render said particle practically incapableof being wetted with water at standard temperature, said particle havingproton affinity sufficient to result in said particle being wetted afterextended contact with boiling water.

DETAILED DESCRIPTION

This invention provides a process for preparing hydrophobic finelydivided oxides of metals and/or oxides of silicon by anchoringhydrocarbon radicals on the surface of the oxide by treatment of theoxides with metal or metalloid compounds which are hydrolyzable andwhich contain OR groups. By means of this invention it is possible toproduce lipophilic fillers entirely notwettable by water, but whichnevertheless possess a high degree of proton affinity. The process ofthis invention is carried out in a very simple manner by a trulytopochemical reaction on the active surfaces of the starting substances.

The ester compound used in the process of this invention has the generalformula:

    Me(OR).sub.4

me is an element of the IVth group of the periodic table. Titanium andsilicon are preferred. R is an alkyl radical having 1-8 carbon atoms, oran aryl radical, such as phenyl or naphthyl. The radicals R in the estercan be independently selected from the alkyl and aryl radicals. Forexample, three of the R's in the ester can be alkyl radicals and thefourth R an aryl radical. Further, each of the three or four R's can bedifferent alkyl radicals.

According to the process of this invention, the ester is intensivelymixed with the oxides to be treated. The oxide particles should have awater content of less than about 1% by weight. Preferably, the watercontent is about 0-0.2% by weight. A particularly preferred ester is atetraethyl-alcosilicate or an oligomer thereof.

In a preferred embodiment of this invention, the ester and oxides aremixed at a pressure of about 1,000 Torr to about 10⁻² Torr, and at atemperature of about 20° - 300° C. Mixing is preferably conducted for afew minutes to about 24 hours. A preferred temperature is about 20° -200° C, while 20° - 35° C is particularly preferred.

In another preferred embodiment of this invention, the oxide particlesare activated prior to mixing with the ester. Activation is accomplishedby heating the oxide particles at about 700° - 1,000° C for less thanabout 60 seconds in a stream of inert gas. An inert gas is one whichdoes not react with the oxide particles. Activaton of the particles inthis manner reduces the time required to obtain hydrophobic products.

In the case of silicic acids or their mixtures with other metal oxides,such as Al.sub. 2 O₃ or TiO.sub. 2, having a water content of more thanabout 1% by weight, the treated products exhibit no hydrophobiccharacteristics. If large quantities of the ester compound are used, thefinal products exhibit insufficient hydrophobic properties. Certainoxides take on water during storage or during production processes. Thewater can be practically removed by the activation step previouslydescribed. See also German Patent Application No. P 17 67 226.3 and P 2004 443.3.

Ester compounds falling within the previously mentioned generic formulacan be used. It is preferred that the ester compound be either gaseousor volatile at the mixing temperature in order to be evenly distributedupon contact with the oxide particles. However, less volatile estercompounds can also be used, such as those in liquid form. The liquid canbe sprayed on the oxide particles by means well known in the art. Itwill of course be understood that mixtures of the various estercompounds can be used.

In a preferred embodiment of this invention, ester compounds areselected, which when mixed with oxide particles, produce a productentirely unwettable by water at ambient temperature within 30 minutes -3 hours. Alkoxy and aroxy silanes and the similar compounds of titaniumhave proven to be particularly suitable. It will also be understood thatmixtures of oxides can be treated with any of the esters.

It is preferred that activation be carried out in a fluidized bed.

The process of this invention can be carried out in almost any desiredmanner. A continuous method of production is preferred. For example,activated silicic acid can be contacted with one or more ester compoundsin a fluidized bed.

In another preferred embodiment of this invention, the oxide particlesare contacted with anhydrous ammonia gas prior to mixing, during mixingor after mixing of the oxides and ester. When the oxides and esters arecontacted in a fluidized bed, and if alcohol which develops during thereaction and ammonia are to be removed, which is generally the case,then the reaction mixture can subsequently be passed through a shorteror longer tarrying zone. The reaction mixture can subsequently be passedthrough a countercurrent fluidized bed in which the removal of ammoniaand alcohol is carried out by means of a stream of inert gas.

The process of this invention will be more clearly understood byreference to the following examples:

Example 1

a. 500 g of a pyrogenic silicic acid (surface area 120 m² /g BET) areplaced in a vessel after activation, and put under a vacuum. 50 g (of10% by weight) tetra-n-butoxy silane are dripped in at room temperaturewithin a few minutes, while it is briskly agitated. After the vacuum isneutralized with N₂, a dry NH₃ stream of gas is introduced into theproduct for 2 - 3 seconds. A sample taken 30 minutes after conductingammonia through it remains completely unwettable even after intensiveshaking with water; the aqueous phase is completely clear.

b. The experiment described in Example 1(a) is carried out in the samemanner, but without the treatment with NH₃.

A sample taken after 3 hours is largely hydrophobic; after intensiveshaking with water the aqueous phase is slightly turbid.

A hydrophobic silicic acid produced according to Example 1(a) is leftstanding in water for 96 hours at ambient temperature. It is shakentwice daily and intensively each time for 5 minutes. At the end of theexperiment, the aqueous phase is still completely clear.

The capability for reaction of the same completely hydrophobic productat ambient temperature is evident by boiling with water because of itssimultaneously existing proton affinity. For example, after 30 minutesin hot water of 100° C, 20% of the originally hydrophobic silicic acidof Example 1(a) passes over into the aqueous phase.

EXAMPLE 2

500 g of a mixed oxide (silicic acid with 1% Al₂ O₃), surface area 80 m²/g BET, with a spectroscopically measured water content of less than0.1%, is put into a vessel, placed under a vacuum, and sprayed for 3minutes with 50 g (of 10% by weight) tetrakis-ethylhexoxy silane, whilebriskly agitated. After the vacuum is neutralized with N₂, a dry streamof NH₃ gas is introduced in the mixture for 3 seconds. Subsequently, theproduct is heated in a rotary flask for 1 hour at a temperature of 100°C.

Afterwards the product is completely hydrophobic. A sample is left for96 hours in water standing at ambient temperature. It was shakenintensively twice daily, each time for 5 minutes. The aqueous phaseremains completely clear for the entire time.

EXAMPLE 3

500 g of a precipitated silicic acid (surface area 150 m² /g BET) aresprayed after activation with 50 g (of 10% by weight) tetra-ethoxysilane at ambient temperature and while being stirred intensively.

After 5 minutes a dry stream of NH₃ gas is conducted through the mixturefor 2 - 3 seconds while stirring.

15 minutes after feeding in the ammonia, the product is no longer wettedby water.

EXAMPLE 4

500 g of a pyrogenic silicic acid (surface area 120 m² /g BET) areplaced in a vessel after activation, put under vacuum, and 75 g (of 15%by weight) butyl titanate (monomer), heated previously to about 70° C,are dripped in within a few minutes at ambient temperature while beingagitated briskly.

After 30 minutes the vacuum is neutralized with N₂, and a dry stream ofNH₃ gas is conducted through the mixture for 2 to 3 seconds.

A sample is taken after 8 hours; even after intensive shaking with waterit remained entirely unwettable; the aqueous phase is still completelyclear after standing for 8 days.

I claim:
 1. A process for preparing hydrophobic finely divided oxideparticles consisting essentially of pyrogenic or precipitated silicicacid particles by chemically bonding hydrocarbon radicals to the surfaceof said particles, said process comprising rendering said particleshydrophobic by intensively mixing said particles with a hydrolysablemetal or metalloid compound containing OR groups, wherein said compoundis an ester of the formula:

    Me(OR).sub.4

wherein Me is an element from group IV of the Periodic Table, and each Ris independently selected from C₁ -C₈ alkyl radicals, an aryl radical oran alkaryl radical; wherein prior to mixing, during mixing, or aftermixing of the oxides and ester, said particles are contacted withanhydrous NH₃ gas; and further wherein said particles have a watercontent less than about 1% by weight.
 2. Process according to claim 1 inwhich said silicic acid particles are in admixture with Al₂ O₃ orTiO.sub.
 2. 3. Process of claim 1 in which Me is titanium or silicon. 4.Process of claim 1 in which mixing is conducted at a pressure of about1000 Torr to about 10⁻² Torr, and at about 20°-300° C for a few minutesto about 24 hours.
 5. Process of claim 4 in which the oxides have awater content of about 0-0.2% by weight.
 6. Process of claim 5 in whichmixtures of oxides are treated with the ester.
 7. Process of claim 4 inwhich the oxide particles are activated prior to mixing with the ester,said activation comprising heating the oxide particles at about700°-1000° C for less than about 60 seconds in a stream of inert gas. 8.Process of claim 7 in which activation is carried out in a fluidizedbed.
 9. Process of claim 8 in which the ester is a tetraalkoxysilane oran oligomer thereof.
 10. Process of claim 4 in which the mixing isconducted at about 20°-200° C.
 11. Process of claim 4 in which mixing isconducted at about 20°-35° C.
 12. Process of claim 9 in which the esteris tetrakis-ethylhexoxy-silane.
 13. Process according to claim 1 inwhich said hydrolyzable metal or metalloid compound is in a gaseousphase.
 14. A process for preparing hydrophobic finely divided oxideparticles consisting essentially of pyrogenic or precipitated silicicacid particles by chemically bonding hydrocarbon radicals to the surfaceof said particles, said process consisting of rendering said particleshydrophobic by intensively mixing said particles at a pressure of about1000 Torr to about 10⁻² Torr, and at about 20° to about 300° C for a fewminutes to about 24 hours with a hydrolysable metal or metalloidcompound containing OR groups, wherein said compound is an ester of theformula:

    ME(OR).sub.4

wherein Me is titanium or silicon, and each R is independently selectedfrom C₁ -C₈ alkyl radicals, an aryl radical or an alkaryl radical,wherein prior to mixing, during mixing, or after mixing of the oxidesand ester, said particles are contacted with anhydrous NH₃ gas; andfurther wherein said particles have a water content less than about 1%by weight.